Dental restorations or prostheses are often made of two or more components, with the individual components providing different characteristics for the restoration. For example, a support structure or frame may provide excellent structural support, and a facing may provide excellent aesthetics. The frame generally is a supporting structure for the dental restoration that provides mechanical stability and usually comprises an interface by which the restoration can be affixed to a prepared tooth of a patient. The facing typically provides for pleasing aesthetic characteristics that gives the restoration the shape and appearance of natural teeth. In addition, both the frame and the facing are shaped to fit well with the adjacent and opposed teeth in a patient's mouth.
In the recent years ceramic materials have been widely used for making high-quality dental restorations because of their good physical, aesthetic and biological properties. These restorations are often manufactured by an automated process, which typically includes:                capturing data representing the shape of a patient's teeth, for example by scanning a plaster model of the patient's teeth or alternatively by scanning the actual teeth in the patient's mouth;        designing the shape of a frame based on the captured data using software, such as computer-aided design (CAD) software; and        manufacturing the frame to correspond to the designed shape, for example, by an automated Computer Numerical Controlled (CNC) machine.        
There are approaches to integrating the steps of capturing, designing and manufacturing in a Computer Integrated Manufacturing (CIM) system. An exemplary CIM system for making frames for dental restorations is available from 3M ESPE AG (Seefeld, Germany) under the trade designation LAVA™.
Although such CIM systems would allow the manufacture of an integrated dental restoration (frame and facing together, in the example mentioned above), it is difficult to provide a single ceramic material that provides both the necessary structural durability and good aesthetics. Therefore the CIM system is normally used to manufacture the frame from a ceramic material that provides the required mechanical durability without regard to its aesthetic properties, after which a final layer or facing is applied to the frame to provide the necessary aesthetic properties. A facing of this type is very often manually prepared by skilled dental technicians, for example by manually applying several layers of a polymeric material or a glass-ceramic material, to provide the appropriate color, translucency, and other properties.
Another common method for manually preparing a facing is the “press over” technique. A frame is manufactured as described above, and manually covered with a wax layer or “wax-up” with an outer surface that corresponds to the desired final shape of the tooth. That wax-up is used to form a pattern for a mold. The mold is then heated in a furnace so that the wax is burned off, and the frame remains as a core in the mold. The space between the core and the interior of the mold is then filled with a molten glass-ceramic material that is, for example, obtained from melting a ceramic pellet in channel or previously molten and poured into the channel, where it flows around the frame and fuses with the frame to form the facing. The restoration may then be removed from the mold, polished as necessary, and provided to the dentist for application to the patient's tooth.
According to a more recent approach, the dental facing is milled out of blocks and adhesively fixed on the dental support structure.
WO 2006/120255 A2 relates to a method for production of a tooth replacement piece using CAD/CAM technology, wherein the tooth replacement piece comprises a skeleton as first component and a facing piece as a second component. Fixing of the first component to the second component can done by using a fixing composition comprising a low melting glass material or organic adhesive.
To adjust and to keep the distance between the support structure and the facing structure it is proposed to mount a distance piece to the inner side of the facing piece.
U.S. Pat. No. 5,346,397 describes a process for making artificial porcelain teeth comprising a joining step of a skeleton part and a facing piece with the help of small spheres of defined diameters in order to obtain a defined spacing between skeleton part and facing part. The small spheres are added into the gap between skeleton and facing so that their diameter rules the distance between them.
U.S. Pat. No. 6,371,762 B1 refers to a ceramic tooth restoration and method for manufacturing. The tooth restoration comprises two neighbouring tooth replacement elements, a connecting member and a glass paste for sintering said connecting members.
WO 2008/144342 A2 describes a method for making a facing for a dental restoration. It is described that a facing precursor can be assembled to a frame using slurry. The slurry used according to a specific example can comprise a glass ceramic powder material and a liquid comprising a certain amount of polyglykol 4000, a certain amount of propandiol and a certain amount of water.
Conventional and commercially available modelling liquids and powders have the defect that they often lead to bite enlargements of the final dental restoration. It has been observed that the mating composition often solidifies before the dental articles are properly attached together.
Thus, there is still room for improvement especially with regard to the requirements to be fulfilled with respect to modern dental materials.